Question

Q14. A student carries a reaction that becomes more and more acidic as it proceeds due to the formation of H3O+. The latter is detrimental to her reaction as it lowers the pH. She, therefore, chooses to run the reaction in a solution that will prevent this. Which of the following should she pick?

A. A mixture of 0.5 M NaOH and 0.5 M HCl

B. A mixture of 0.5 M HCl and 0.5 M NaCl

C. A mixture of 0.5 M H2CO3 and 0.5 M NH4Cl

D. A mixture of 0.5 M H2CO3 and 0.5 M NaHCO3

E. None of the above

The correct answer is D, I just want to know the reason why. Thumb up guaranteed if the explanation is adequate.

Answer 1

As mentioned in the question, the reaction became more and more acidic due to formation of H₃O⁺ and causes the lowering of pH. In order to prevent from this to happen, one has to use the buffer solution.

Buffer solutions are those which resist the change in pH on addition of external acid or base and thus maintaining stable pH. Buffers are usually composed of weak conjugate acid-base pairs that is either a mixture of weak acid and salt of its conjugate base or weak base and salt of its conjugate acid.

Out of different options given;

A) A mixture of equivalent amounts of strong base and strong acid i.e. 0.5 M NaOH and 0.5 M HCl will result in formation of neutral salt NaCl which can not resist the change in pH

B) A mixture of 0.5 M HCl and 0.5 M NaCl will also be not able to from buffer since it is a combination of strong acid and neutral salt.

C) A mixture of 0.5 M H₂CO₃ and 0.5 M NH4Cl

Carbonic acid (H₂CO₃) is a weak acid that dissociates to form H⁺ and HCO₃^- and salt of its conjugate base would be NaHCO3. Since NH₄Cl is not a salt of conjugate base of weak acid i.e. H₂CO₃ this combination is not suitable as buffer.

D) A mixture of 0.5 M H2CO3 and 0.5 M NaHCO3

Carbonic acid (H₂CO₃) is a weak acid that dissociates to form H⁺ and HCO₃^- . The H₂CO₃ is present in combination with the sodium salt of its conjugate base which is NaHCO₃. This is the combination suitable to act as buffer.